Also a magazine should be "disposable". With modern materials, phenolics and spring set myths busted a pre-packaged and loaded magazine packed and issued in crates with seven mags to a bandoleer and 18 bandoleers to a crate. Thus a crate would resupply a basic 9 man squad.
Very do-able, but don't necessarily think about a magazine extending from the weapon at a 90º angle like most of today's magazines, but consider a inline magazine coaxial to the barrel/action/stock like that of the Calico, which in a 9mm pistol/SMG design offers a 50 or 100 round capacity. The company had been working on a .223 version when they relocated from California due to that state's *assault weapons* ban.
A Hill-design magazine atop the receiver is also possible, or a multiple tube feed as per the South African Neosted bullpup pump shotgun is possible, too. How would you feel about having your ammo in 10-round tubes, easily carried in something along the lines of a bowman's quiver for arrows, with the weapon taking four or five of them simultaneously, and not extending at all when in the prone position.
Take a look at the old Civil War butt-fed Spencer carbine....
